Lightguides generally consist of a very thin transparent fiber which has an inner core region and an outer cladding region, the cladding having a lower refractive index than the core. The refractive index difference between the cladding and the core may be abrupt or graded. The cladding is typically protected by a resilient jacket.
In using lightguides for lightwave communications, it is desirable to be able to couple light carried in the cores of two or more fused silica glass lightguide elements together in an efficient manner. This can be accomplished by either fusion or lapping processes. The fusion process involves bringing the fibers in close contact and then fusing them together by the application of heat while pulling to reduce the size of the assembly. The degree of optical coupling is then determined by the core-to-core spacing and the interaction length. Heat fusion of this type is difficult to control because of the relatively high temperatures involved, the small size and heat capacities of the members to be fused, and the tendency of the glass to change to a rounded geometry as soon as it becomes sufficiently softened to fuse uniformly. In the lapping process, the jacket is stripped away and the exposed fiber segment is mounted in a holder, such as a grooved block. Then the side of the cladding is abraded away by lapping to expose the core, so that it may be placed in close proximity to another, similarly exposed core for optical coupling. This approach is time consuming and permits only rather limited coupling configurations.